Droplet generation devices are used in a variety of analysis and characterization applications such as fractional charge searches in particle physics, aerosol characterization in chemistry, and cell-sorting techniques in biology. Such devices are also used in material deposition applications such as inkjet printing and microfabrication.
Droplet generators used in conventional analysis applications typically consist of a vertical dropper situated above an analysis chamber. The dropper generates a vertical stream of droplets which is optically analyzed as it falls through the analysis chamber. Depending on the application, the droplets may be subjected to electric fields, air currents, or other perturbations. For examples of droplet generators used for fractional charge searches see the article by Savage et al. in Phys. Lett. 167(B4):481 (1986), as well as the M.S. thesis of Joyce, San Francisco State University (1985).
Desirable properties for droppers used in analysis applications include: ability to adjust droplet sizes either electronically or by replacing a minimal amount of hardware, ease of replacement for droplet nozzles, ability to generate a large number of analyzable droplets, and chemical inertness of all tubing (including nozzle).
It is important to have a chemically non-reactive dropper. Typical droplet generators contain metal or plastic parts, and may therefore chemically react with or biologically contaminate the fluid under study. Zoltan, in U.S. Pat. No. 3,683,212, shows how to make a dropper that is chemically inert, using glass or other tubing that is necked down to form a nozzle through which droplets are ejected. However, manufacturing such a nozzle requires a highly skilled artisan, and no two nozzles will be identical. Furthermore, the tapering in such a nozzle is generally so gradual that the nozzle tends to clog easily.
Using a single nozzle for drop generation is desirable since it allows a significant reduction in total system cost, and in particular in nozzle replacement costs. Typical prior art single-nozzle systems are limited to generating a single stream of droplets, however, and therefore can only provide a limited amount of data within a given field of view of a camera.
Droplet generators used in conventional material deposition applications such as inkjet printing typically consist of a linear array of vertical nozzles situated closely above the deposition target (e.g. a sheet of paper). Each nozzle ejects droplets at a fixed location as the target moves relative to the array. The dropper array may be used to generate two-dimensional droplet arrays. The requirement for placing the nozzles in close proximity to the target makes imaging the impact points of the droplets relatively difficult. For information on droplet generators used for inkjet printing see for example U.S. Pat. Nos. 5,124,716, 3,683,212, and 5,619,234.